Would a direct IST-SYD be the worlds longest flight?
SYD-IST is 9,293 miles.
I actually think TK may go for a IST-MEL routing - SYD is served up pretty well, and there are much more people with Turkish/Greek heritage in MEL, who may be interested in IST as a destination/connection point, compared to SYD.
IST-MEL is still a chunky 9,083 miles.
According to
Non-stop flight - Wikipedia, the free encyclopedia both of these would be the longest non-stop commercial flights in the world, after SQ stops its SIN-EWR routing.
Having a world record non-stop route could be considered a "prestige point" for TK, (helpful marketing spot for an up-and-coming airline anxious to build a global brand). Just like having built the world's record tallest tower was a "prestige point" for other emerging economies like Malaysia or UAE.
However, I very strongly doubt that they would do this route non-stop.
* It is difficult to build a reliable clientele as hardly many Turks ever visit Australia for tourism. Besides the obvious cost of a flight, visa requirements are substantially onerous and may take several weeks, if not months. Most prefer travel to Russia nearby, and Thailand/Bali for exotic travel. Business links are also not as strong.
* TK is operated more as a privately owned carrier, than the hobby horse of an oil-rich kingdom. This would be a very costly marketing exercise.
Thus, I suspect their routing would allow for a streamlined refuelling stop in Jakarta or KL, which would increase route flexibility and would introduce some welcome competition to QF on these routes.